Astronomers Unveil 2017 OF201: A Discovery Challenging Outer Solar System Models

In May 2025, astronomers led by Sihao Cheng at the Institute for Advanced Study announced the discovery of 2017 OF201, a trans-Neptunian object (TNO) with an exceptionally elongated orbit, challenging existing models of our solar system's outer regions.

The identification of 2017 OF201, a 700-kilometer-wide body with a 25,000-year orbit extending beyond 1,600 astronomical units (AU), suggests a more complex and populated outer solar system than previously understood, prompting a reevaluation of theories regarding distant celestial bodies.

The discovery was made through an extensive analysis of archival data spanning seven years, utilizing observations from telescopes in Chile and Hawaii. The research team, comprising Cheng and Princeton University graduate students Jiaxuan Li and Eritas Yang, identified 2017 OF201 in images dating back to 2011. This meticulous approach underscores the value of existing datasets in uncovering new celestial objects.

2017 OF201's orbit is highly eccentric, with a perihelion (closest point to the Sun) of approximately 44.9 AU and an aphelion (farthest point) extending beyond 1,600 AU. This extreme trajectory places it among the most distant known objects in the solar system. Its estimated diameter of about 700 kilometers suggests it may qualify as a dwarf planet, similar to Pluto.

The object's unique orbital characteristics challenge existing models of the solar system's outer regions, particularly the hypothesis of a ninth planet, often referred to as Planet Nine. The discovery suggests a more complex and populated outer solar system than previously understood, prompting a reevaluation of theories regarding distant celestial bodies.

Sihao Cheng, the lead researcher, is a Martin A. and Helen Chooljian Member in the School of Natural Sciences at the Institute for Advanced Study. He holds a Ph.D. in astrophysics from Johns Hopkins University and has a background in cosmology, stellar physics, and extrasolar planets. The Institute for Advanced Study, located in Princeton, New Jersey, is renowned for its contributions to theoretical research and has been home to many prominent scholars, including Albert Einstein and John von Neumann.

Future observations are planned to further study 2017 OF201 and other potential planets in the solar system. The object's extreme orbit may provide valuable insights into the dynamics of our solar system.

The discovery of 2017 OF201 underscores the potential for numerous undiscovered bodies in the distant reaches of our solar system, prompting a reevaluation of our understanding of its structure and dynamics.